Service Worker: Add tests for same-scope, new script registration

Tests for Chromium patch:
https://codereview.chromium.org/506043002/

BUG=398355

Review URL: https://codereview.chromium.org/480943002

git-svn-id: svn://svn.chromium.org/blink/trunk@180948 bbb929c8-8fbe-4397-9dbb-9b2b20218538